Brett- Kimbrough makes shields that fit onto thier spur gears that helps to keep dirt and carpet fibers out of your diff. They are really cheap and just snap on. I used to have to clean it every race day or it would feel just as you described. Also use very little diff lube just enough to coat the balls all the excess just attracts dirt. I only have to clean them every other week now.

Here is a link they are under accessories.

http://www.kimbrough-products.com/